One of the problems that currently exists in the field of laser technology is insufficient quality of radiation from edge-emitting laser diodes. It is known that light beams emitted from edge-emitting laser diodes have a complicated structure. These beams are asymmetric and exhibit different divergences in the emitter plane (slow axis) and in the plane perpendicular to the emitter plane (fast axis). Although fast-axis divergence is much greater, the wavefront is close to a diffraction-limited spherical shape, and the beam can be easily collimated with a spherical lens. On the other hand, the slow-axis beam structure can be very complicated, and collimation is very difficult, if even possible.
The radiation structure of the aforementioned type significantly complicates formation of desired beams and their collimation and focusing on a target, as well as coupling into optical fibers. A common solution to the above problem demands the use of anamorphous optics, such as special collimators for fast and slow axes, special focusing optics, etc. However, precision collimators of this type are expensive, and this limits their use in practice. Designs of collimators for beams propagated in the direction of slow and fast axes are known and described in numerous patents, for example, U.S. Pat. Nos. 4,687,285; 5,940,564; 6,031,953, and European Patent EP No. 864,892.
It is understood that the above problem is even greater for laser diodes with wide emitters, i.e., with emitters having a high ratio of emitter width to emitter height. The driving force behind widening the emitter area is the desire to increase output power without damaging the output face of the laser diode. An example of such laser diodes that recently appeared on the market is a device having an emitter width greater than 100 microns (slow axis) and a height of less than 1.5 microns (fast axis). The output power of these diodes exceeds several watts and may reach tens of watts, and the structure of their radiation has a complicated multimode nature that leads to high divergence of the output beam. Conventional approaches to the solution of the above problem with respect to the wide-aperture edge-emitting laser diodes do not allow for forming single transverse mode beams without significant loss of power and increase in weight or size. Therefore, advantages inherent in optical laser devices are not used to their full potentials for wide-aperture edge-emitting laser diodes.
In view of the above, the problem of improving optical characteristics such as mode composition, beam divergence in the direction of slow and fast axes without noticeable reduction in optical power, and, hence, brightness, is an extremely important task in laser technology.
An innovative method of controlling the optical parameters of a light beam such as direction of light propagation, change of phases, spectral dispersion, etc., as proposed in U.S. patent application Ser. No. 12/011,453 filed Jan. 28, 2008, is the use of planar optical waveguides with quasi continuous change in the refractive index. This approach is known as digital planar holography (DPH), a new technology recently developed for fabricating miniature components for integrated optics. The essence of DPH technology is the embedding of digital holograms calculated by a computer inside a planar waveguide.
The DPH allows for light propagation in the hologram plane rather than in the perpendicular direction and results in a long interaction path. Benefits of a long interaction path are well known for volume/thick holograms. On the other hand, planar configuration provides easy access to the surface, where the hologram should be embedded, enabling a simple fabrication process.
As known, light is confined in waveguides by a refractive index gradient and propagates in a core layer surrounded with a cladding layer. Materials for core/cladding layers should be selected so that the core refractive index Ncore is greater than that of the cladding layer Nclad: Ncore>Nclad. Cylindrical waveguides (optical fibers) allow for one-dimensional light propagation along the axis. Planar waveguides, which are fabricated by sequentially depositing flat layers of transparent materials with a proper refractive index gradient on a standard wafer, confine light in one direction (axis z) and permit free propagation in two other directions (axes x and y).
A lightwave propagating through the waveguide core extends to some degree into both cladding layers. If the refractive index is modulated in the wave path, the light from each given wavelength can be directed to a desirable point.
DPH technology can be used for designing and fabricating holographic nanostructures inside a planar waveguide, thus providing conditions for light processing and control. There are several ways of modulating the core refractive index, the simplest of which is engraving the required pattern by means of nanolithography. Modulation is created by embedding a digital hologram on one of the core/cladding interfaces or on both of them. Standard lithographical processes can be used, making mass production straightforward and inexpensive. Nanoimprinting is another viable method for fabricating DPH patterns. Each DPH pattern is computer-generated and is customized for a given application. The consists of numerous nanogrooves, each ˜100 nm wide, positioned so as to provide maximum efficiency for a specific application.
The devices are fabricated on standard wafers. While the total number of nanogrooves is huge (=106), the typical size of DPH devices is on a scale of millimeters.
DPH structure can be described as a digital planar hologram that comprises an optimized combination of overlaid virtual subgratings, each of which is resonant to a single wavelength of light.